Process for cleaning an article, process for coating an article, and device therefor

ABSTRACT

The surface of an article with a metallic base body is cleaned. A plasma comprising electrically positively charged ions is generated, and the ions are accelerated toward the article, so that they come into contact with the base body for cleaning purposes. To do this, an electron beam is directed onto the base body. The outgoing flux of electrons which come into contact with the base body is controlled by the base body being connected to a reference potential via a switch of at a fixed, adjustable or regulated frequency.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of copending InternationalApplication No. PCT/EP99/07997, filed Oct. 21, 1999, which designatedthe United States.

BACKGROUND OF THE INVENTION

[0002] Field of the Invention

[0003] The invention relates to a method of cleaning the surface of aproduct, in particular a component of a thermal machine, such as a gasturbine blade or a heat shield element, which has a metallic base body.The invention also relates to a method of applying a coating, inparticular a thermal barrier coating, to an article which can be exposedto a hot aggressive gas and has a metallic base body. The invention alsorelates to a device for cleaning a product which has a substrate chamberin which there is a substrate guide and which has a substrate holder.U.S. Pat. No. 5,238,752 describes a thermal barrier coating system withan intermetallic bond coat. The thermal barrier coating system isapplied to a metallic base body, in particular to a Cr-Co-steel for anaircraft engine blade. An intermetallic bond coat, in particular ofnickel aluminide or a platinum aluminide, is applied directly to thismetallic base body. This bond coat is adjoined by a thin ceramic layerof aluminum oxide, to which the actual thermal barrier coating, inparticular of yttrium stabilized zirconium oxide, is applied. Thisceramic thermal barrier coating of zirconium oxide has a rod-likestructure, the rod-like columns being oriented substantiallyperpendicular to the surface of the base body. The intention of this isto improve the ability to withstand cyclic thermal loads. The thermalbarrier coating is deposited on the base body by means of an electronbeam PVD (Physical Vapor Deposition) process, zirconium oxide beingvaporized from a metal oxide body by an electron beam gun. The processis carried out in a corresponding device, in which the base body ispreheated to a temperature of approximately 950° C. to 1000° C. Duringthe coating operation, the base body is rotated at a constant rate inthe jet comprising the metal oxide.

[0004] An electron beam PVD process for producing a ceramic coating isalso described in U.S. Pat. No. 5,087,477. The ceramic coating producedin this case has a layer thickness of between 250 and 375 μm.

[0005] To provide good adhesion between the coating and the base body,it is advantageous for the base body to be cleaned prior to the coatingoperation. It has become known from British patent specifications GB 2323 855 and GB 1 447 754 to clean a product which is to be coated priorto the coating. The cleaning in those cases takes place by means of asputtering process, in which firstly a plasma is generated and thepositive ions of the plasma are accelerated toward the base body. Thedevice for cleaning the base body is integrated in the device forcoating the base body. In order for the base body to be heated to asuitable coating temperature, the base body is heated with the aid of anelectron beam. There is provision for it to be possible to switchbetween the heating phase by means of the electron beam and the cleaningphase by means of ion sputtering. To do this, it is necessary for therelationship of potential between the base body and the electron sourceor the positive ions of the plasma to be adjusted or controlled. To dothis, GB 2 323 855 provides for the base body to be connected to avoltage source, in order to set the base body to a suitable potential.In GB 1 447 754, a voltage source and a monitoring device are alsoprovided, in order to be able to influence the relationships ofpotential between base body, electron source and plasma. Therefore, toset a suitable relationship of potential, according to both literaturesources an active voltage supply is required.

SUMMARY OF THE INVENTION

[0006] The object of the present invention is to provide a method ofcleaning an article, in particular a component of a gas turbine, with aview to a coating process that is to be carried out on the article whichovercomes the above-noted deficiencies and disadvantages of the priorart devices and methods of this general kind. It is a further object ofthe invention to provide a method of coating an article with aprotective coating and/or a thermal barrier coating. An additionalobject of the invention is to provide a device for cleaning and/orcoating a product.

[0007] With the above and other objects in view there is provided, inaccordance with the invention, a method of cleaning a surface of anarticle having a metallic base body, the method which comprises:

[0008] generating a plasma with electrically positively charged ions,accelerating the ions towards the article, and bringing ions intocontact with the base body for cleaning the base body;

[0009] directing an electron beam onto the base body; and

[0010] controlling an outgoing flow of electrons coming into contactwith the base body by connecting the base body to a reference potentialvia a switch at a given frequency, which may be fixed preset,adjustable, or regulated.

[0011] In other words, the object relating to a process for cleaning thesurface of a product which has a metallic base body is achieved by thefact that a plasma with electrically positively charged ions isgenerated and the ions are accelerated toward the product, so that theycome into contact with the base body for cleaning purposes, an electronbeam being directed onto the base body, and that the outgoing flux ofelectrons which come into contact with the base body is controlled as aresult of the base body being connected to a reference potential via theswitch with a fixedly preset, adjustable or regulated frequency.

[0012] With this process, the surface of the product, in particular acomponent in a plasma, undergoes preliminary cleaning in such a mannerthat the adhesion of layers which are to be vapor-deposited issignificantly improved compared to a process involving thermal cleaning.The latter may, for example, lead to gases escaping.

[0013] Compared to the processes which are known from the prior art, theprocess described here has the crucial advantage of being significantlysimpler and therefore also less susceptible to faults.

[0014] This is primarily due to the fact that the connection of the basebody to the reference potential which is, for example, frame potentialor ground potential, can be switched on and off makes it easy tosuitably adjust the relationship of potential between base body andplasma and the electrons of the electron beam. Therefore, the electricalpotential of the base body is controlled by means of the connection tothe reference potential. The outgoing flux can in this case becontrolled between a maximum value and a minimum value, the minimumvalue preferably being zero, i.e. there is no outgoing flux ofelectrons. In the latter case, the electrons do not flow out and anelectron build-up is produced around the product, which negativelycharges the product. In the presence of the plasma, the positivelycharged ions are accelerated toward the product; they come into contactwith the product at a parameter-dependent velocity. Contaminants presenton the surface of the product are removed by means of a pulse exchangewith this surface.

[0015] On the other hand, if the outgoing flux is set to a maximumvalue, i.e. the switch is closed and the base body is connected to thereference potential, for example, frame potential, the electrons canflow out of the electron beam without obstacle. Consequently thepositive ions of the plasma are not accelerated toward the base body.Therefore, when the switch is closed, the product is substantially onlyexposed to the electron beam, which heats the product. Therefore, byactuating the switch it is simple to switch over between a cleaningphase and a heating phase. The switch can be suitably actuated with afixedly preset, adjustable or regulated frequency.

[0016] The outgoing flux of electrons preferably takes place via anelectrical outgoing line, which is alternately opened and closed bymeans of the switch. This outgoing line produces a current path which isconstantly switched between passing and blocking. The alternatingswitching between passing and blocking of the current path can takeplace at a constant, possibly temporally variable frequency. Alternatingswitching allows alternating charging and discharging of the product, asa result of which, in the presence of a gas, an alternating voltagedischarge (plasma) can be ignited or maintained. In this way, it ispossible to continuously clean the component.

[0017] The frequency at which the outgoing flux of electrons iscontrolled may in this case lie between a few hertz and frequencies upto the megahertz range, in particular, the frequency may beapproximately 50 kHz or approximately 27 MHz. The high-frequencyswitching has the crucial advantage that, at suitable high frequencies,the cleaning effect is not dependent upon the component geometry.Therefore, the high- frequency switching allows reliable and inparticular homogenous cleaning of the product.

[0018] There is a potential difference, i.e. an electric voltage,between the plasma and the base body, which potential difference can beinfluenced, in particular, set, by suitably controlling the outgoingflux of electrons from the base body. This potential difference producesa bias voltage in a range from approximately 100 V to approximately 1000V. This bias voltage may be selected in such a way that the formation ofthe plasma, in particular as a result of an alternating voltagedischarge, can be ignited and maintained.

[0019] Preferably the bias voltage between the electrically positivelycharged ions of the plasma and the base body is determined and, ifappropriate, can be used to control the outgoing flux of the electrons.It is thus also possible to carry out a regulating process between biasvoltage and outgoing flux of the electrons with the view to achievingthe most expedient cleaning of the product possible depending on thetype of metallic base body (geometric shape, metallurgical composition,etc.). The bias voltage is in this case measured and displayed as atemporal mean.

[0020] In accordance with an added feature of the invention, the plasmais generated by an electron beam, in particular by the electron beamwhich is directed onto the base body and can also be used, inter alia,to heat the product. The electron beam may in this case be of fan-shapedor cone-shaped design as an electron beam fan or cone, in order to beable to irradiate a large area of the base body and to be able togenerate a sufficiently large volume of plasma. It is also possible forthe plasma to be generated in a separate process and for the plasmawhich has already been ionized to be passed into the vicinity of theproduct. The gas from which the plasma is formed is preferably an inertgas, in particular a noble gas, such as argon. This ensures that noundesirable chemical reactions take place on the surface of the product,in particular the base body, but rather the product is simply cleaned.As an alternative, it is also possible to use a reactive gas, inparticular hydrogen, to form the plasma. When using hydrogen, the plasmais also able to remove oxide on the product by means of oxidation toform water.

[0021] The product is preferably rotated about an axis of rotationduring the cleaning in which positively charged ions come into contactwith the product. The result is uniform cleaning and heating of theproduct even in the event of complex geometries.

[0022] To enable the cleaning to be carried out as quickly andeffectively as possible, in a particularly advantageous embodiment, theproduct is heated before the cleaning operation. The heating causes alarge proportion of the contaminants on the surface and in the productto vaporize or gasify. The gasification of contaminants situated in thebody of the product is in this context particularly advantageous with aview to the properties of the component. As a result, the cleaning phaseby means of ion firing (sputtering) which follows this thermal cleaningphase becomes more effective. Therefore, preheating of the product priorto the sputtering is of considerable significance primarily from aneconomic viewpoint, not least because of the time saved. This embodimentwith preheating is not restricted to the configuration with an outgoingflux of electrons passing via a switch to the reference potential, forexample frame potential. Rather, it is independent of the particular wayin which the potential level of the product is set.

[0023] During the cleaning, the product is preferably simultaneouslyheated to a temperature which is greater than or equal to the coatingtemperature, which in particular lies above 800° C. Heating to atemperature above the coating temperature has the beneficial effect thatthere is relatively little evolution of gases during the subsequentcoating at a lower temperature. This heating can be achieved by means ofthe electron beam, which simultaneously serves to generate acontrollable negative potential of the base body.

[0024] The process for cleaning and, if appropriate, simultaneouslyheating the product is preferably integrated in a process for coatingthe product with a protective layer, in particular a thermal barriercoating. According to the invention, the second object, relating to aprocess for coating a product, is therefore achieved by the fact thatthe product undergoes prior cleaning by a plasma, in the mannerdescribed above, prior to the actual coating process.

[0025] The process for producing a thermal barrier coating is preferablycarried out as an electron beam physical vapor deposition (EB-PVD)process or as a reactive gas flow sputtering process as described, forexample, in international PCT publication WO 98/13531 A1.

[0026] Within the context of the overall coating process, the producthas preferably already been preheated prior to the cleaning, to atemperature which in particular is higher than the actual coatingtemperature of the product, which is over 800° C. As has already beenstated, this increases the effectiveness of the cleaning by means of ionsputtering. The heating takes place, for example, with the aid of theelectron beam and/or a further heating device. After the cleaning, theproduct is heated to the coating temperature; this is also to beunderstood as meaning that heating to the coating temperature hasalready taken place during the cleaning, so that after the cleaning theproduct is at the coating temperature. Preferably, preheating, cleaningand coating immediately follow one another, in which case the preheatingalready produces an initial cleaning action and in which case during theactual cleaning it is possible to switch between sputtering mode andheating phase (by means of electron firing). The fact that the threeprocesses of preheating, cleaning and coating immediately follow oneanother ensures that the product is always held at a sufficiently hightemperature level.

[0027] In accordance with an additional feature of the invention, theactual cleaning process is carried out in a chamber, referred to belowas the substrate chamber. This may be a preheating chamber of a coatinginstallation, the actual coating chamber itself, or a separate chamberwhich is designed specifically for the cleaning. To generate theelectron beam which is used to electrically charge the base body, it ispossible to use an electron beam gun, which is likewise used to carryout the coating process or is accordingly designed only for cleaning orheating. An electron beam gun of this type may have an electron beamcapacity of up to 150 kW with an acceleration voltage of up to 35 kV.

[0028] With the above and other objects in view there is also provided,in accordance with the invention, a device for cleaning an article,comprising:

[0029] a housing defining a substrate chamber;

[0030] a substrate guide disposed in said substrate chamber;

[0031] a substrate holder for holding an article connected to saidsubstrate guide in a mechanically fixed and electrically insulatedmanner;

[0032] an electrical outgoing line connected to said substrate holder,and a switch connected in said electrical outgoing line for selectivelyconnecting said substrate holder to a reference potential; and

[0033] an electron beam gun for generating an electron beam directedonto the article.

[0034] In other words, the object relating to a device for cleaning aproduct, in particular a component of a gas turbine, is achieved by adevice which has a substrate chamber in which a substrate guide with asubstrate holder for holding the product is provided, the substrateguide being connected in a mechanically fixed but electrically insulatedmanner to the substrate holder, and it being possible to connect thesubstrate holder to reference potential via an outgoing line and bymeans of a switch arranged in the outgoing line. The reference potentialmay in this case, for example, be frame potential or ground potential.

[0035] In accordance with another feature of the invention, the switchis connected to a control device for controlling the alternating openingand closing of the switch. This makes it possible to actuate the switchwith a fixedly preset, adjustable or regulated frequency. The substratechamber may in this case be the actual coating chamber of a coatinginstallation, a preheating chamber of a coating installation or aseparate chamber.

[0036] The outgoing line is preferably connected to a current- and/orvoltage-measuring device, so that the electron current passing throughthe outgoing line and a bias voltage between the base body and a plasmawhich is present in the substrate chamber can be measured. The plasmaitself is preferably generated by firing an electron beam from aelectron beam gun onto the base body. The electron beam gun may in thiscase be arranged inside the substrate chamber or outside this chamberand may be specifically designed for firing the base body, for examplefor heating purposes. It is also possible to use an electron beam gunwhich is used to fire at a coating target in order to produce a coatingon the base body. The current- and/or voltage-measuring device ispreferably connected to the control device.

[0037] Furthermore, to regulate the switching frequency of the switch,it is preferable to provide a regulator for which a desired value ispreset. A desired frequency value for the regulator may in this case beestablished, for example, by means of a desired current value and/or adesired voltage value.

[0038] Preferably, the substrate chamber is a coating chamber of acoating installation and the substrate holder and the substrate guide,after cleaning of the product, also simultaneously serve to hold theproduct during the coating with a protective layer, in particular athermal barrier coating. During the cleaning, the product is held in thesubstrate holder, which is electrically insulated with respect to thesubstrate guide, which serves as the anode. Together with the outgoingline, the substrate holder forms a current path which is controllable,i.e. switchable. The substrate holder is therefore likewise brought tothe same potential as the substrate guide, preferably to groundpotential (earth). The parameters such as bias voltage, velocity of thepositive plasma ions, etc. can be set, by means of the frequency of theswitchable current path which is formed via the electrical outgoingline, in such a way that a particularly good cleaning action is producedaccording to the particular product used.

[0039] The product is preferably a component of a thermal machine, inparticular of a gas turbine, such as a stationary gas turbine used inthe power plant sector or a component of an aircraft engine turbine. Theproduct may in this case be designed as a heat shield of a combustionchamber or as

[0040] a turbine blade, a turbine rotor blade or a turbine guide vane.

[0041] In accordance with a further feature of the invention, theprotective layer, in particular the thermal barrier coating, is aceramic layer. It may include zirconium oxide (ZrO₂) or another ceramicmaterial which is suitable for use at high temperatures, in particular ametal oxide. A zirconium oxide is preferably partially or completelystabilized with yttrium oxide (y₂O₃) or another oxide of a rare earthelement.

[0042] In accordance with yet a further feature of the invention, thebase body is a metallic substrate. Nickel-base and/or cobalt-base alloyssuch as those which are described, for example, in U.S. Pat. No.4,405,659, inter alia, are particularly suitable for applications athigh temperatures with corresponding demands imposed on resistance tocorrosion.

[0043] An adhesion promoter layer is arranged between base body andthermal barrier coating. This adhesion promoter layer may consist of analloy of iron, cobalt and/or nickel with chromium, aluminum, yttrium.Instead of, or in addition to yttrium, one of the elements from groupIIIb of the periodic system, including the actinides and the lanthanidesand, in addition, or as an alternative, rhenium may be included.Yttrium-containing alloys of this type are described in the literatureunder the designation “MCrAlRe” alloy. Alloys which containsignificantly more rhenium than yttrium may be referred to as “MCrAlRe”alloys. An oxide layer, in particular a layer of aluminum oxide,chromium oxide and/or gallium oxide, may be provided between theadhesion promoter layer and the thermal barrier coating. An oxide layerof this type may already have been applied as an oxide or may form overthe course of time as a result of subsequent oxidation (thermally grownoxide, TGO).

[0044] Other features which are considered as characteristic for theinvention are set forth in the appended claims.

[0045] Although the invention is illustrated and described herein asembodied in a process for cleaning a product, process for coating aproduct and device therefor, it is nevertheless not intended to belimited to the details shown, since various modifications and structuralchanges may be made therein without departing from the spirit of theinvention and within the scope and range of equivalents of the claims.

[0046] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodimentswhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047]FIG. 1 is a perspective view of a turbine rotor blade;

[0048]FIG. 2 is a cross section through a turbine blade;

[0049]FIG. 3 is a partial sectional view of a thermal barrier coatingsystem of the turbine blade of FIG. 2; and

[0050]FIG. 4 is a diagrammatic section through a coating installationfor coating a turbine blade with thermal barrier coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is seen, as a representative ofthe article of manufacture 1, a rotor blade of a turbine, in particularof a gas turbine. The rotor blade 1, which is made from iron, cobaltand/or nickel, has a blade root 14, by means of which it can be securedin a non-illustrated rotatable turbine shaft. The blade root 14 isadjoined by the actual blade root region, which extends from a leadingedge 7 to a trailing edge 8 via a pressure side 9 on one side, and asuction side 10 on the other side. Cooling channels 13 for carrying acooling medium, in particular cooling air, are provided in the actualblade region. The blade region has a surface 4 with differently curvedsurface regions.

[0052]FIG. 2 illustrates a cross section through a gas-turbine blade—again representing the article of manufacture—around which a hot gasflows 16 during its use in a gas turbine. In cross section, the turbineblade 1 extends from a leading edge 7, via a pressure side 9 and asuction side 10, to a trailing edge 8. The turbine blade 1 is formedfrom a substrate or base body 2, in the interior of which a plurality ofcooling channels 13 for carrying cooling air are provided. The entiresurface 4 of the turbine blade 1 is coated with a thermal barriercoating 5.

[0053]FIG. 3 diagrammatically depicts the structure of a thermal barriercoating system 15. The thermal barrier coating system 15 is applied tothe base body 2. Directly adjacent to the base body 2, it has anadhesion promoter layer 11, which is adjoined by an oxide layer 12 andthe actual thermal barrier coating 5, which is on top of the oxide layer12. The adhesion promoter layer 11 may be a known alloy of the typeMCrAlY or MCrAlRe. The oxide layer 12 may substantially comprise analuminum oxide or may alternatively or additionally include metaloxides, such as chromium oxide or gallium oxide. The choice of adhesionpromoter layer 11 and of oxide layer 12 naturally depends on thematerial of the base body 2 and on the thermal barrier coating 5 to beapplied. The latter may, for example, consist of partially stabilizedzirconium oxide. The thermal barrier coating 5 has a fine structure withceramic columns 6 which are oriented substantially perpendicular to thesurface 4 of the base body 2. The ceramic columns 6 each have a meancolumn diameter D which, for a layer thickness of the thermal barriercoating 5 of approximately 100 μm to 200 μm, lies in the range between0.5 and 5 μm, preferably below 2.5 μm.

[0054]FIG. 4 shows a diagrammatic longitudinal section through a device20 for cleaning a article 1, which is integrated in a coating device forapplying a thermal barrier coating 5 to the article 1, in particular toa gas turbine blade. The device 20 has a substrate chamber 24, whichserves as a coating chamber. A suitable sub-atmospheric pressure(vacuum) can be established in the chamber 24, and a gas, in particularan inert gas, is introduced. Pumps, which are not shown and to which thesubstrate chamber 24 is connected via a pump outlet 36, are provided forthe purpose of evacuating the chamber 24 and to build the vacuum in thechamber 24. A substrate guide 26, which extends along an axis ofrotation 32 and is designed, for example, as a hollow cylindrical tube,is introduced into the substrate chamber 24 via an introduction chamber38. The substrate guide 26 is adjoined by a substrate holder 22 which ismechanically fixedly connected to the substrate guide. The article 1 isheld rotatably, and optionally also pivotably, in the substrate holder22. The substrate guide 26 is electrically insulated from the substrateholder 22 by insulation 27. The substrate guide 26 is grounded outsidethe substrate chamber 24.

[0055] The substrate holder 22 is connected to an electrical outgoingline 28 which, for example, is guided through the substrate guide 26. Ameasuring device 31A, 31B, in particular a current-measuring device 31Afor measuring an electric current I and a voltage-measuring device 31Bfor measuring an electrical bias voltage U, is arranged in theelectrical outgoing line 28. Furthermore, a switch 29, which can becontrolled by a control device 30, is provided in the outgoing line 28.The switch 29 may be designed as a mechanical or electronic switch or asa suitable control mechanism. Its essential function is that ofcontrolling the electric current I flowing through the outgoing line 28.The outgoing line 28 is also connected, outside the substrate chamber24, to a reference potential, which may be frame or ground. The switch29 and the measuring devices 31A, 31B are connected to a control device30, by means of which the switch 29 can be controlled, so that theswitch 29 opens and closes at a frequency which can be preset by meansof the control device 30. The frequency or on/off duration (duty factor)used for the opening and closing may be effected as a function of thebias voltage U determined by means of the measuring device 31A, 31B. Thefrequency may also be fixed by means of a desired current value I*and/or a desired voltage value U*, the desired current value I* and/orthe desired voltage value U* being input into the control device 30 andbeing compared to the actual values U, I in the control device 30. Thecontrol signals for the switch 29 are passed from the control device 30to the switch 29.

[0056] An electron beam gun 18, which generates an electron beam 19, isprovided on the substrate chamber 24, above the substrate holder 22. Theelectron beam 19 may, in this case, as shown, be of fan-shaped orcone-shaped design as an electron beam fan or electron beam cone. Usingthe grounding of the substrate guide 26 and the controllable grounding(reference potential) of the substrate holder 22, the electron beam 19is guided toward the article 1. On the way to the article 1, theelectron beam 19 causes the gas situated in the substrate chamber 24,for example, an inert gas such as argon, to be ionized. As a result, aplasma 21 is formed in the vicinity of the article 1. The electronswhich come into contact with the article 1 from the electron beam 19 aredischarged via the outgoing line 28 when the switch 29 is closed.

[0057] When the switch 29 is open, there is a build-up of electrons infront of the article 1, i.e. a negatively charged cloud of electronsaround the article 1, with the result that the positively charged ionsof the plasma 21 are accelerated toward the article 1. The positive ionswhich have been accelerated in this way come into contact with thearticle 1 and thus cause contaminants on the article 1 to be removed.

[0058] The constant change between an open and closed state of theswitch 29 therefore leads to a frequency-dependent alternating chargingand discharging of the article 1, so that an alternating voltagedischarge (plasma formation) is ignited or maintained. This allowscontinuous cleaning of the article 1, and surface activation prior to acoating operation. High frequency switching is preferably establishedvia the switch 29 and the control device 30 in such a manner that acleaning action which is independent of the geometry of the article 1 isachieved. This leads to particularly effective and homogeneous surfacecleaning.

[0059] A coating target 23, for example made from zirconium or zirconiumoxide, is arranged in the substrate chamber 24, which at the same timerepresents the coating chamber of a coating installation, at a lowerlevel than the article 1. A further electron beam gun 25 for generatinga further electron beam 35 is provided in the substrate chamber 24. Tocarry out the actual coating of the article 1, the substrate chamber 24has a feed 33 for an oxygen-containing gas 34, so that additionaloxidation for a metal-ceramic thermal barrier coating can be achieved.The article 1 is, for example, a gas turbine blade or a heat shieldelement with a protective layer, in particular with a thermal barriercoating made from a ceramic. The coating target 23 is grounded duringthe actual coating operation. The electron beam 35 is diverted towardthe coating target 23 (as shown in dashed lines), and then comes intocontact with the coating target 23, where it causes the material of thecoating target 23, in particular zirconium or zirconium oxide, to bevaporized. The material which has been vaporized in this way flowstoward the article 1, where it is deposited, if appropriate withsimultaneous oxidation, as a protective layer (thermal barrier coating).The article 1 is in the process rotated about the axis of rotation 32 asa result of the entire substrate guide 26 being rotated about the axisof rotation 32 by means of a non-illustrated motor. The rotation of thearticle 1 about the axis of rotation 32 is also effected during thecleaning operation by means of plasma ions. The plasma discharge andtherefore the ion firing of the substrate 1 can also be maintainedduring the coating, so that interim cleaning is achieved.

[0060] In the example illustrated, the entire coating process can becarried out in the substrate chamber 24. Preferably, in a first processstep, the article 1 is heated, without the presence of a gas which formsa plasma 21, by means of an electron beam 19 to a temperature which liesabove the actual coating temperature. This advantageously leads to priorthermal cleaning on account of the vaporization/gasification ofcontaminants. In a second process step, in which gas which forms theplasma 21 is now present, renewed firing using the electron beam 19takes place, with the result that simultaneous cleaning of the article 1by means of the plasma 21 and heating of the article 1 by means of theelectron beam 19 are achieved. On account of the prior thermal cleaning,this final cleaning by means of ion firing (sputtering) can be carriedout significantly more quickly and more effectively. After the cleaningof the article 1 has ended, this product has been heated to the coatingtemperature, and electrons are fired on to the coating target 23, withthe result that the protective layer 5, in particular a thermal barriercoating, is applied to the article 1. The cleaning of the article 1 bymeans of the plasma 21 renders the adhesion of the thermal barriercoating to the article 1 particularly superior.

I claim:
 1. A method of cleaning a surface of an article having ametallic base body, the method which comprises: generating a plasma withelectrically positively charged ions, accelerating the ions towards thearticle, and bringing ions into contact with the base body for cleaningthe base body; directing an electron beam onto the base body; andcontrolling an outgoing flow of electrons coming into contact with thebase body by connecting the base body to a reference potential via aswitch at a given frequency.
 2. The method according to claim 1 , whichcomprises selecting the given frequency from the group consisting of afixed preset frequency, an adjustable frequency, and a regulatedfrequency.
 3. The method according to claim 1 , which comprisesalternately opening and closing the switch to define an outgoing flux ofelectrons in an electric outgoing line connected to the base body. 4.The method according to claim 1 , which comprises controlling anoutgoing flux of electrons at a frequency in a range from a few Hz to afew MHz.
 5. The method according to claim 1 , which comprisescontrolling an outgoing flux of electrons at a frequency ofsubstantially 50 kHz.
 6. The method according to claim 1 , whichcomprises controlling an outgoing flux of electrons at a frequency ofsubstantially 27 MHz.
 7. The method according to claim 1 , whichcomprises controlling an outgoing flux of electrons such that a biasvoltage of substantially between 100 V and 1000 V is established betweenthe electrically positively charged plasma and the base body.
 8. Themethod according to claim 1 , which comprises determining a bias voltagebetween the electrically positively charged ions of the plasma and thebase body.
 9. The method according to claim 1 , wherein the generatingstep comprises generating the plasma with the electron beam.
 10. Themethod according to claim 1 , which comprises forming the plasma with agas selected from the group consisting of inert gas and reactive gas.11. The method according to claim 1 , which comprises forming the plasmawith noble gas.
 12. The method according to claim 1 , which comprisesforming the plasma with argon.
 13. The method according to claim 10 ,which comprises forming the plasma with the reactive gas hydrogen. 14.The method according to claim 1 , which comprises heating the articleprior to cleaning.
 15. The method according to claim 14 , whichcomprises heating the article by irradiation with electrons.
 16. Themethod according to claim 1 , which comprises, concurrently withcleaning the article, heating the article to a coating temperature. 17.The method according to claim 16 , which comprises heating the articleto a coating temperature of over 800° C.
 18. The method according toclaim 1 , which comprises providing a gas turbine component as thearticle and cleaning a surface of the gas turbine component.
 19. Themethod according to claim 18 , which comprises selecting the gas turbinecomponent from the group consisting of a turbine blade and a heat shieldelement.
 20. The method according to claim 1 , which comprises rotatingthe article about an axis of rotation.
 21. A method of coating anarticle, which comprises cleaning the article with the method accordingto claim 1 , and subsequently coating the article with a protectivelayer with a physical vapor deposition process.
 22. The method accordingto claim 21 , which comprises, prior to the cleaning step, heating thearticle to a given temperature, and after the cleaning step and prior tothe coating step, heating the article to a defined coating temperature.23. The method according to claim 22 , wherein the coating temperaturelies above 800° C., and the given temperature lies above the coatingtemperature.
 24. A device for cleaning an article, comprising: a housingdefining a substrate chamber; a substrate guide disposed in saidsubstrate chamber; a substrate holder for holding an article connectedto said substrate guide in a mechanically fixed and electricallyinsulated manner; an electrical outgoing line connected to saidsubstrate holder, and a switch connected in said electrical outgoingline for selectively connecting said substrate holder to a referencepotential; and an electron beam gun for generating an electron beamdirected onto the article.
 25. The device according to claim 24 ,wherein said substrate holder is configured to hold a component of a gasturbine.
 26. The device according to claim 24 , which further comprisesa control device connected to said switch for controlling an alternatingopening and closing of said switch.
 27. The device according to claim 24, which further comprises a measuring device connected to said outgoingline for measuring one of a current and a voltage.
 28. The deviceaccording to claim 27 , which further comprises a control deviceconnected to said switch for controlling an alternating opening andclosing of said switch, and a measuring device connected to saidoutgoing line and to said control device for measuring one of a currentand a voltage.
 29. The device according to claim 24 , which comprises aregulator connected to said switch, said regulator being preset to adesired value for regulating a switching frequency of said switch.